6,754 research outputs found
Ferromagnetic materials in the zinc-blende structure
New materials are currently sought for use in spintronics applications.
Ferromagnetic materials with half metallic properties are valuable in this
respect. Here we present the electronic structure and magnetic properties of
binary compounds consisting of 3d transition metals and group V elements viz.
P, Sb and As in the zinc-blende structure. We demonstrate that compounds of V,
Cr and Mn show half metallic behavior for appropriate lattice constants.
By comparing the total energies in the ferromagnetic and antiferromagnetic
structures, we have ascertained that the ferromagnetic phase is stable over the
antiferromagnetic one. Of the different compounds studied, the Cr based systems
exhibit the strongest interatomic exchange interactions, and are hence
predicted to have the highest critical temperatures. Also, we predict that VAs
under certain growth conditions should be a semiconducting ferromagnet.
Moreover, critical temperatures of selected half metallic compounds have been
estimated from mean field theory and Monte Carlo simulations using parameters
obtained from a {\it ab-initio} non-collinear, tight binding linearized
muffin-tin orbital method. From a simple model, we calculate the reflectance
from an ideal MnAs/InAs interface considering the band structures of MnAs and
InAs. Finally we present results on the relative stabilities of MnAs and CrSb
compounds in the NiAs and zinc-blende structures, and suggest a parameter space
in substrate lattice spacings for when the zinc-blende structure is expected to
be stable.Comment: 7 pages, 6 figure
Atomistic spin dynamics of the CuMn spin glass alloy
We demonstrate the use of Langevin spin dynamics for studying dynamical
properties of an archetypical spin glass system. Simulations are performed on
CuMn (20% Mn) where we study the relaxation that follows a sudden quench of the
system to the low temperature phase. The system is modeled by a Heisenberg
Hamiltonian where the Heisenberg interaction parameters are calculated by means
of first-principles density functional theory. Simulations are performed by
numerically solving the Langevin equations of motion for the atomic spins. It
is shown that dynamics is governed, to a large degree, by the damping parameter
in the equations of motion and the system size. For large damping and large
system sizes we observe the typical aging regime.Comment: 18 pages, 9 figure
Universal distribution of magnetic anisotropy of impurities in ordered and disordered nano-grains
We examine the distribution of the magnetic anisotropy (MA) experienced by a
magnetic impurity embedded in a metallic nano-grain. As an example of a generic
magnetic impurity with partially filled -shell, we study the case of
impurities imbedded into ordered and disordered Au nano-grains, described in
terms of a realistic band structure. Confinement of the electrons induces a
magnetic anisotropy that is large, and can be characterized by 5 real
parameters, coupling to the quadrupolar moments of the spin. In ordered
(spherical) nano-grains, these parameters exhibit symmetrical structures and
reflect the symmetry of the underlying lattice, while for disordered grains
they are randomly distributed and, - for stronger disorder, - their
distribution is found to be characterized by random matrix theory. As a result,
the probability of having small magnetic anisotropies is suppressed below
a characteristic scale , which we predict to scale with the number of
atoms as . This gives rise to anomalies in the
specific heat and the susceptibility at temperatures and
produces distinct structures in the magnetic excitation spectrum of the
clusters, that should be possible to detect experimentally
Analytic continuation by averaging Pad\'e approximants
The ill-posed analytic continuation problem for Green's functions and
self-energies is investigated by revisiting the Pad\'{e} approximants
technique. We propose to remedy the well-known problems of the Pad\'{e}
approximants by performing an average of several continuations, obtained by
varying the number of fitted input points and Pad\'{e} coefficients
independently. The suggested approach is then applied to several test cases,
including Sm and Pr atomic self-energies, the Green's functions of the Hubbard
model for a Bethe lattice and of the Haldane model for a nano-ribbon, as well
as two special test functions. The sensitivity to numerical noise and the
dependence on the precision of the numerical libraries are analysed in detail.
The present approach is compared to a number of other techniques, i.e. the
non-negative least-square method, the non-negative Tikhonov method and the
maximum entropy method, and is shown to perform well for the chosen test cases.
This conclusion holds even when the noise on the input data is increased to
reach values typical for quantum Monte Carlo simulations. The ability of the
algorithm to resolve fine structures is finally illustrated for two relevant
test functions.Comment: 10 figure
A Model for Predicting Productivity in Subgrade Preparation of Forest Roads by Excavator
The effect of terrain factors on productivity in subgrade preparation by excavator was studied. The data, collected in a follow-up time study of 57 road sections, was analyzed using multiple linear regression. A prediction model that has soil moisture class and boulder frequency as independent variables was derived. The results also show that productivity varies considerably among operators. It is also apparent that the effect of the terrain is partly levelled out as the quality requirements for the performance of the subgrade are normally adjusted to the terrain conditions. It is suggested that, within a certain region, a fairly simple model can be sufficient for practical use in road network planning
A theoretical analysis of the chemical bonding and electronic structure of graphene interacting with Group IA and Group VIIA elements
We propose a new class of materials, which can be viewed as graphene
derivatives involving Group IA or Group VIIA elements, forming what we refer to
as graphXene. We show that in several cases large band gaps can be found to
open up, whereas in other cases a semimetallic behavior is found. Formation
energies indicate that under ambient conditions, sp and mixed sp/sp
systems will form. The results presented allow us to propose that by careful
tuning of the relative concentration of the adsorbed atoms, it should be
possible to tune the band gap of graphXene to take any value between 0 and 6.4
eV.Comment: 5 pages, 4 figures. Transferred to PR
Simulation of a spin-wave instability from atomistic spin dynamics
We study the spin dynamics of a Heisenberg model at finite temperature in the
presence of an external field or a uniaxial anisotropy. For the case of the
uniaxial anisotropy our simulations show that the macro moment picture breaks
down. An effect which we refer to as a spin-wave instability (SWI) results in a
non-dissipative Bloch-Bloembergen type relaxation of the macro moment where the
size of the macro moment changes, and can even be made to disappear. This
relaxation mechanism is studied in detail by means of atomistic spin dynamics
simulations.Comment: 8 pages, 12 figures, submitted to PR
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